1. Field of the Invention
The invention relates generally to surface acoustic wave (SAW) devices and more specifically, to SAW oscillators.
2. Description of the Prior Art
In the prior art, the so-called SAW devices have found a number of uses. Basically, these devices rely on the generation and detection of surface acoustic waves, also known as RAYLEIGH waves, in a piezoelectric material. Although the piezoelectric phenomenon has been known and used to provide stable signal delay device filters and oscillators, the exploitation of the effect had, until comparatively recently, been largely confined to so-called bulk wave applications.
Relatively recently however, the refinement of photo lighographic processing (photoetch techniques) particularly as applied to micro circuits, has made possible the SAW interdigital transducer. Such devices have been described in the technical and patent literature, for example, in U.S. Pat. application 493,673, filed Aug. 1, 1974, entitled: "Video Processor". That U.S. patent application describes the application of SAW technology to a particular type of video processing and is assigned to the assignee of the present application.
A more directly pertinent prior art document, in that it relates to SAW oscillators specifically, is the technical paper entitled: "The Surface Acoustic Wave Oscillator-A natural and Timely Development of the Quartz Crystal Oscillator", by Meirion Lewis of the Royal Radar Establishment, Malvern, Worcs., UK. That paper was published in the Proceedings of the 28th Annual Symposium on Frequency Control, May 1974, and is distributed by the National Technical Information Service of the U.S. Department of Commerce.
In the aforementioned technical paper, the use of the SAW transducer as the feedback element in an oscillator is shown. The paper also suggests ways of producing frequency control or frequency modulation of the SAW oscillator. The relative difficulty of effecting frequency modulation of such a device can be appreciated as several circuit approaches are suggested by Lewis. These approaches include incorporation of a phase shift network in the amplifier circuit of the oscillator loop, use of varactors in a phase shift network, etc.
Piezoelectric devices, and particularly oscillators employing piezoelectric elements, are noted for their stability, and the same benefits accrue to SAW oscillators of the type to which the present invention applies, as generally applied to bulk effect devices. By bulk effect devices, in this case, it is intended to refer to the crystal oscillator of the type well known and widely used in the radio arts for many decades of time.
It will be realized that the introduction of other circuit elements, such as varactors, etc., must inherently deteriorate the stability advantages enjoyed by unmodulated SAW oscillators, because the frequency determination function is partly usurped from the piezoelectric element. Moreover, the additional complication tends to tarnish the attractiveness of the SAW oscillator, which is inherently a very inexpensive and highly satisfactory device capable of being efficiently manufactured in sizes compatible with micro-circuitry.
The manner in which the present invention provides a unique approach to an arrangement for frequency modulation of a SAW oscillator with very little additional structure, with relatively little impact on inherent stability, and with inherent linearity of oscillator frequency as a function of the applied modulating or controlling signal, will be evident as this description proceeds.